Nowadays severe demands are made on fluid-actuated control systems, for brevity also called fluid medium systems in what follows, and particularly on systems of such type designed for automated or automatic transmissions in motor vehicles. Besides special requirements in relation to low weight along with great reliability, long life and low maintenance, in passenger cars in particular vibrations and noise caused by device components of the fluid medium system must be avoided if possible or at least reduced as much as possible. In addition, special requirements exist in relation to the actuation speed of control elements to be actuated, since a traction force interruption caused by a gear change should be as brief as possible. At the same time, the fluid medium system should be as inexpensive as possible to produce and assemble and should consume little energy in operation.
Thus, until now, it has been usual in fluid-actuated transmissions, following a control action, i.e., a change in the position of at least one fluid-actuated control element, to switch off the pressure on the pressure-actuated control element(s) or at least to reduce it to a lower pressure level sufficient to maintain the desired position of the pressure-actuated element(s). This relieves the stress on the pressure medium lines and other pressurized components of the pressure medium system and the control elements, and so enables a reduction of the loading of the entire fluid medium system, which can then correspondingly be designed to be less strong and lighter, and/or to have a longer service life. Furthermore, if the system is subjected to high pressure only when control actions are actually taking place, this has advantages in relation to the electrical energy needed for the actuation of the electrically operated valves that produce the pressure, and advantages in relation to fluid loss rate if there is a leak in the system components concerned.
During a control action, even control elements that are not switched or moved during the control action are generally still acted upon by pressure throughout the entire switching duration, including a post-switching period, in order to prevent or reduce any possible undesired position change and, in this way, they are fixed in their position, secure against any possible unforeseen displacement caused by hardly avoidable local and time-related pressure fluctuations.
In the present document, unless expressly otherwise stated, the term “control action” is understood to mean an action of the fluid-actuated control system in which the position of at least one control element is changed by way of control commands of the control device in such manner that the desired effects are produced on the system to be controlled or regulated. In this context the position of the “control element” is understood to mean the displacement of a movable piston in a cylinder. Several actions that take place simultaneously or at least in close time proximity are regarded as a single control action for these purposes.
Accordingly, the actuation of a control cylinder of a piston-cylinder arrangement whose piston can be coupled to a clutch release lever or a shift claw displacement mechanism or a gearwheel mounted to move on a shaft, constitutes a control action if its result or at least its intention is to produce a desired change in the position of the clutch lining, an engagement or disengagement of the claw clutch, or to bring two gear wheels into mutual engagement. Inasmuch as the closing of the clutch, an engagement of the claw clutch or a displacement of the gearwheel on the shaft, during a gear change, take place virtually simultaneously, these actions can be taken together as a single control action “engage X gear”. Control elements which are actuated in correspondence with the above description will be referred to as “switched” control elements in what follows.
In addition, however, the term “control action” should also be understood to mean cases in which a control element is actuated in order to fix one or more control elements in their position. “Fixing” of a control element is understood to mean that it is acted upon with the aim of keeping its position unchanged, or holding it fixed in its position or, if need be, moving it a certain distance in the direction toward a position it occupied before or one that is desired for it. The distance being substantially smaller than the distance between adjacent functional positions of the control element. Such cases include the action of pressure by control elements intended to bring a piston that has bounced back a short way back into contact with an abutment.
For example, a sensor can determine that a movable control-element piston has moved away from its intended position because of vibrations or external forces, or would be so moved unless this is counteracted by the corresponding action of pressure by a pressurized working fluid. Control elements, which are actuated in accordance with the above description, will be referred to as “non-switched” or “unswitched” control elements in what follows.
In the context of a fluid medium system with several control elements, it has until now sometimes been deemed advantageous, when a control action is to be carried out to apply a specified pressure to the pressure system as a whole by way of a main valve for a standardized time interval consisting of a positioning time and a post-switching time, whereby if the valves associated with the control elements are set in advance or at the same time, the switched control element(s) or their moving parts are brought to the desired position while the un-switched control elements or their moving parts are held fixed in their positions by the action of the pressure.
Often, however, the control elements are connected, via control valves fitted near them, to a pressure medium delivery line that is constantly under a standardized operating pressure during operation and to a pressure medium outflow line which is under lower pressure or unpressurized so that, to actuate a control element for example by the action of valves, this control element is exposed selectively to the operating pressure for a standardized time. In such cases, all the valves associated with the non-switched control elements as well are actuated correspondingly, in order to fix those control elements in their position. In the case of pneumatic systems, of course, the pressure medium outflow line can be replaced by a simple venting option to the surroundings.
Although the pressurization even of non-switched control elements as described is basically reasonable and makes a substantial contribution to the functional reliability of the system, this procedure in the form so far applied is associated with the disadvantages described below.
Because of the simultaneous or at least almost simultaneous pressurization of all the switched and un-switched control elements with fluid under pressure, the fluid flow required for a short time is relatively large. This is particularly the case in pneumatic fluid systems and in hydraulic fluid medium systems as well although to a lesser extent.
Because of the different dead times of valves and control elements and the different geometries of the various fluid lines, there may be a small time-spread of the moments of action, but this is very slight and, as a so-termed “dirt effect”, requires no means for the optimization of the fluid medium system directed against it.
As a direct result of this, it is either necessary to provide a relatively bulky and heavy reservoir container for a working fluid under high pressure or the pressure medium source must be in the form of a pressure medium pump designed to have correspondingly high power. Both variations result in increased weight and higher costs.
Even if an appropriate reservoir container is, in any case, provided, for example on account of safety considerations, related to an emergency running ability of the fluid medium system in the event of a failure of the pressure medium pump, the pressure medium lines to the individual control elements and, in particular main pressure medium lines for the supply of several control elements, must be designed large enough so that they generate only an acceptably low flow resistance against the necessary large fluid medium flows. This increases the necessary cross-sections and leads to limitations in relation to the positioning of the pressure medium lines, for example in a motor vehicle with limited free space and many restrictive conditions for the way these lines run.
A problem similar to the problem of the large pressure medium flow required for a short time as outlined above, concerns the electrical control of the valves associated with the control elements. Since these have to be actuated simultaneously to the positions desired in each case, then when the valves have an approximately equal dead time it follows that they must also receive actuating signals simultaneously. With the electrically actuated valves used customarily this results in a substantial power peak of the electrical consumption which can interfere with other sensitive systems unless counteracted by elaborate measures. Moreover, simultaneous energizing of the valves means that at least some of the valves are operated for an unnecessarily long time, which wastes energy and increases the loading of the valve.
The simultaneous action on all the control elements with fluid under pressure also produces a comparatively large pressure pulse in the system, which not only substantially stresses all the pressure delivery elements of the fluid medium system, but can also result in mechanical vibrations and/or excitation of the system or parts of it into resonant oscillations. Furthermore, there may be noise emissions which are undesirable precisely in the context of motor vehicles and in particular passenger cars.
Against this background, the purpose of the present invention is to propose a control method for a fluid medium system, for example an automated or automatic transmission of a motor vehicle, which does not suffer from the disadvantages outlined above or at least only so to a considerably lesser extent. In particular, it must be possible to fix the shift elements according to need and only for a short time.